This invention relates to knock controls for spark ignited internal combustion engines of the type which sense knock and adjust an engine operating parameter such as ignition timing in order to prevent such knock from exceeding trace levels. Such systems are known in the art as shown in U.S. patents such as Harned et al U.S. Pat. No. 4,002,155, issued Jan. 11, 1977, West U.S. Pat. No. 4,106,477, issued Aug. 15, 1978, West et al U.S. Pat. No. 4,111,035, issued Sept. 5, 1978, Kearney et al U.S. Pat. No. 4,276,861, issued July 7, 1981, Chen et al U.S. Pat. No. 4,364,260, issued Dec. 21, 1982, Brandt U.S. Pat. No. 4,384,473, issued May 24, 1983, and Oh U.S. Pat. No. 4,424,706, issued Jan. 10, 1984. In addition, such systems have been produced on motor vehicles in this and other countries for a number of years.
Such systems generally include a vibration or detonation sensor adapted to sense knock-induced vibrations of the engine and provide an electrical signal thereof to knock measuring circuitry. Eventually, in almost all such systems, a processed signal including knock pulses is compared to a reference voltage level with the output indicative of knock. In a signal amplitude controlled system, some means must be provided to generate a reference level higher than the noise level over which knock peaks may be detected, in spite of varying signal strength due to such factors unrelated to knock intensity as inherent sensor output level and path characteristics between the source of knock vibrations and the sensor. Some of such systems, including most of the systems described in the above-mentioned patents, derive the reference level from the signal itself in a noise channel. Other such systems provide automatic gain control (AGC) for the signal to standardize the average signal level and supply a fixed reference level.
In either case, it is the background noise level which is desired for control; and the effect of knock pulses themselves, either in the noise channel or in the AGC control signal, is a distortion which should be minimized. In the case of the noise channel, apparatus responsive to the knock pulse output of the comparator to inhibit an increase in the noise channel voltage was described and claimed in the above--mentioned patent to West et al U.S. Pat. No. 4,111,035; and a variation was shown in the above-mentioned patents to Kearney et al and to Brandt. However, no such apparatus appears in the prior art showing AGC controls in knock control systems, such as those shown in the patents to King et al U.S. Pat. No. 4,153,020, issued May 8, 1979, Kashimura et al U.S. Pat. No. 4,425,891, issued Jan. 17, 1984, Kobayashi U.S. Pat. No. 4,463,722, issued Aug. 7, 1984, Hattori et al U.S. Pat. No. 4,476,709, issued Oct. 16, 1984, and Kobayashi U.S. Pat. No. 4,481,924, issued Nov. 13, 1984.
In addition, the apparatus shown in the above-mentioned patents to West et al, Kearney et al and Brandt employs a capacitive current integrator in the noise channel with a switch responsive to the comparator output to completely cut off the current input to the integrator. One result of this is that, for the time the integrator input current is cut off, the charge on the capacitor may discharge through a resistive discharge path to ground and thus decrease the noise channel voltage. Although this has not created any problems in knock control during normal vehicle operation of the noise channel type of system, it might be desirable, in the case of an AGC system, for the integrator input current to continue, but at a lower rate, to stabilize the gain of the AGC amplifier.